This invention relates generally to air-in-line detectors. Specifically, the present invention relates to an air-in-line detector which uses a series of optical sensors spaced axially along a fluid flow line and having selectively different light sensitivities and selectively different optical path arrangements to provide a sequence of signals for determining whether an air bubble is present in the fluid flow line. More specifically, the present invention relates to an air-in-line detector which is insensitive to changes in the level of ambient light and which can determine the presence of air bubbles in a fluid line regardless of the opacity or opalescence of the fluid. This invention is particularly, but not exclusively, suited for use with an I.V. infusion pump or controller to determine the presence of air bubbles in the I.V. administration fluid flow line.